The present invention relates generally to electronic data processing systems and more particularly to electronic security systems for controlling access to, or operation of, cars, homes, or the like.
There are numerous types of security systems and various coding schemes which are more or less useful to secure or operate the locks of cars, homes or the like. Some of these systems are quite simple and handy but offer only a minimum of protection and security, whereas others are highly sophisticated with electronic or otherwise coded devices. Such coded devices may include a magnetic card strip inserted into a card reader for reading the information coded on the card as disclosed in U.S. Pat. No. 4,316,083 to Harris et al., a punch card inserted into a card reader for reading the information coded on the card as disclosed in U.S. Pat. No. 3,803,551 to Jordan, a key equipped with microelectronic components inserted into a receptacle for interrogation of the information stored on the key as disclosed in U.S. Pat. No. 4,326,125 to Flies, or a coded device such as a card brought within the range of a field generated by the security system as disclosed in U.S. Pat. No. 4,354,189 to Lemelson.
All of these systems have their drawbacks. For example, if a punch card is lost or stolen, it is a simple matter to produce counterfeit punch cards which negate the effectiveness of the security system. Even with electronic key-type devices, should one be lost or stolen, the security code or codes retained by the key-type device can be ascertained by supplying power and inputting clock pulses or the like to the key-type device. Once the security codes are developed, counterfeit key-type devices can be produced thus rendering the security system useless.
The problem of impairing the effectiveness of the security system because of a lost or stolen punch card, key-type device, or other device used to gain access is well recognized. One solution for electronic key-type devices is to provide key-type devices which are programmable thus allowing the security codes to be changed.
Another proposed solution for electronic key-type devices is to provide a buffer between the electronic components carrying the coded information and the output ports of the key-type device as disclosed in U.S. Pat. No. 4,412,216 to Mole et al. In the patent to Mole et al. a shift register is used as the buffer. However, once the appropriate signal is found which causes the security code to be loaded into the shift register, the security code can easily be learned.
Designers of security systems are therefore faced with the competing design criterion of, on the one hand, designing a system offering maximum security against unauthorized duplication of the device used to gain access while on the other hand desiring a device which is simple, easy to fabricate, and easy to use.